The disclosures of the publications, patents, and published patent specifications referenced in this application are hereby incorporated by reference into the present disclosure to more fully describe the state of the art to which this invention pertains.
The need for rechargeable batteries with long cycle life, rapid charge capacity, and high energy density for devices such as mobile telephones, portable computers and other consumer electronic devices continues to grow. Rechargeable batteries, such as those based on lithium metal anodes and solid electroactive sulfur-containing cathode active materials, provide one approach to meet this need. For example, U.S. Pat. Nos. 5,529,860, 5,601,947, and 5,690,702 to Skotheim et al., and U.S. Pat. No. 6,201,100 to Gorkovenko et al., describe electroactive sulfur-containing cathode active materials and lithium/sulfur batteries using these sulfur-containing cathode active materials.
However, one problem encountered in electrochemical cells based on lithium and sulfur-containing cathode active materials is limited cycle life, i.e., the number of recharging that the battery can accept before the battery is no longer able to maintain acceptable levels of charge capacity, such as 50-80% of the initial capacity of the battery.
It has been shown that the charge conditions may directly affect the lithium surface morphology in recharging lithium secondary cells with lithium metal anodes and with transition metal oxide cathodes. It is believed that lithium surface morphology created in the lithium deposition process is one important factor in determining cycle life. For example, Aurbach et al., in J. Electrochem. Soc., 1988, 145, 1421-1426, report a much lower cycle life for Li—Lix MnO2 cells, with lithium metal anodes under fast charge rates (1.25 mA/cm2) compared with slow charge rates (0.3 mA/cm2).
It has also been shown that discharge rates may affect the cycle life of rechargeable batteries. For example, it has been reported that high discharge rates for lithium cells result in longer cycle life than low discharge rates. For example, Saito et al. report, in J. Power Sources, 1998, 72, 111-117, that for LiNV2O5—P2O5 cells, low rate discharging (0.5 mA/cm2 results in a higher surface area for a lithium metal anode and in much lower cycle life than high rate discharging (5.0 mA/cm2).
Lithium sulfur battery continues to suffer from several problems that have hindered its broad commercialization. One of the obstacles is the solubility of the lithium polysulfides (PS) (Li2Sx, 2<x<8) generated during the charge/discharge processes. These higher order PS derived from the reduction of elemental sulfur are highly soluble in organic electrolytes and can be fully reduced at the lithium metal anode. PS can also accumulate at the surface of the carbon cathode and be further reduced to lower order PS, such as Li2S2 or Li2S. The insulating nature of these lower order PS blocks the electron pathway on the cathode. This is detrimental for the long-term operation of the battery. Also, by the end of complete discharge elemental sulfur converts to Li2S. Over a repeated charge-discharge cycles, dissolution and deposition of PS over the surface of cathode results in morphological changes. Gradually, due to the morphological changes, sulfur losses contact with the cathode and become inactive. Hence, to enhance the cycle life of the lithium sulfur battery, it is important to minimize the formation of Li2S.
There is a need in rechargeable lithium metal batteries for both long cycle life and rapid charge times, and for charging methods that maximize the cycle life while shortening charge times. There is also a need for charging regimes designed for rechargeable batteries comprising sulfur-containing cathodes. The present invention addresses the need for rapid charge times while at the same time achieving long cycle life for rechargeable batteries comprising sulfur-containing cathodes.
It would be an advancement in the art to provide a simple method to determine a remaining capacity of an electrochemical cell. Further, it would be an advancement in the art to correlate a state of charge profile with a reference in conjunction with a degradation model to terminate a charging process.